This invention relates to electrically conductive organosiloxane elastomers. More particularly, this invention relates to storage-stable silver-filled organosiloxane compositions yielding cured electrically conductive elastomers that retain their electrical properties for extended periods of time. The reduced variation of contact resistance and volume resistivity with time exhibited by the elastomers are attributed to the manner in which the silver particles are processed prior to being incorporated into the curable organosiloxane composition.
Elastomers, gels and resins prepared from curable organosiloxane compositions containing finely divided silver particles exhibit high levels electrical conductivity and are therefore used in specialized applications that require materials exhibiting heat resistance, flex resistance, and electrical conductivity.
Japanese Patent Application Laid Open [Kokai or Unexamined] Number Hei 3-170581 [170,581/1991] teaches an electrically conductive silicone rubber composition comprising an organopolysiloxane containing at least 2 alkenyl radicals in each molecule, an organohydrogensiloxane containing at least 2 silicon-bonded hydrogen atoms in each molecule, a platinum-containing hydrosilation catalyst, and silver particles.
The silver particles used to prepare electrically conductive silicone rubber are typically classified as chemically reduced silver, electrolytically reduced, and atomized silver. Chemically reduced silver is prepared by reacting an aqueous silver nitrate solution with a reducing agent such as hydrazine, formaldehyde or ascorbic acid. Electrolytically reduced silver is prepared from aqueous silver nitrate solutions by electrolytic deposition at the cathode. Atomized silver particles are prepared by spraying molten silver maintained at a temperature of at least 1,000xc2x0 C. into water or an inert gas.
Silver particles are available in the form of granules, flakes, dendrites or amorphous particles. Silver flake is preferably used because it yields silicone rubbers with particularly high electrical conductivities.
Japanese Laid Open Patent Application No. 59/170,167 discloses a method for preparing gold- or silver-filled inks by blending an organic solvent with a powdered form of gold or silver that has been treated with the combination of a methylhydrogen polysiloxane and an amino-functional silicone oil.
U.S. Pat. No. 5,227,093, which issued on Jul. 13, 1993 teaches increasing the electrical conductivity of elastomers and other products prepared from curable organosiloxane compositions containing finely divided silver particles by treating the silver particles with a fatty acid ester prior to blending the silver particles with the other ingredients of the curable composition.
Japanese Laid Open Patent Application No. 03/49,105 describes electrically conductive particles suitable for use with adhesives. The particles exhibit diameters of from 1 to 20 microns and comprise a core of a high polymer on which is deposited a layer of silver followed by a layer of gold. During blending with the ingredients of a curable organosiloxane composition the particles are treated with a silane coupling agent selected from the group consisting of gamma-methacryloxypropyltrimethoxysilane, gamma-mercaptopropyltrimethoxysilane and gamma-chloropropyltrimethoxysilane.
Several problems are associated with the electrically conductive silicone rubber composition taught in Japanese Patent Application Laid Open Number Hei 3-170581 and the other publications mentioned in the immediately preceding paragraphs. When silver in a flake form is used as a filler, it has been found that during storage of the composition not only do the silver particles separate from the composition, but the curability of the composition declines with the passage of time to the point that the composition ultimately becomes uncurable.
Another problem associated with the prior art conductive silicone rubber compositions that the large variations in contact resistance and volume resistivity that occur with the passage of time in cured elastomers prepared using the curable compositions described in this patent publication. This phenomenon renders the rubber unsuitable for use for the continuous electrical connection of electrically conductive elements.
The present inventors have been able to determine that one cause of the variation of electrical properties of silver-filled organosiloxane elastomers with time is the low affinity of silver particles for the other ingredients of the curable composition used to prepare the elastomer.
The present inventors have also been able to confirm that the curability of electrically conductive silicone rubber compositions declines with elapsed time due to the presence of residues of the lubricant that is present during grinding of the silver particles. These lubricant residues remain on the surface and/or in the interior of the particles.
One or more of the following lubricants typically have been used with silver particles during grinding of the particles: saturated and unsaturated higher fatty acids such as lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, arachidic acid, and behenic acid; metal soaps such as aluminum laurate, aluminum stearate, zinc laurate, and zinc stearate; higher aliphatic alcohols such as stearyl alcohol; the esters of higher aliphatic alcohols and carboxylic acids; higher aliphatic amines such as stearylamine; higher aliphatic amides; and polyethylene waxes.
The present inventors attempted to remove the lubricant remaining on the surface of lubricant-treated silver flake by subjecting the silver particles to repeated washings with organic solvents. The washings did not provide a satisfactory suppression of the timewise variation in the curability of electrically conductive silicone rubber compositions containing this type of silver as a conductive filler.
The present inventors have also confirmed that a decline in adhesion and affinity between the cured silicone elastomer and the silver particles contribute to the timewise variations in, respectively, the contact resistance and volume resistivity of electrically conductive silicone rubbers prepared by curing silver-filled electrically conductive silicone rubber compositions.
As the result of extensive investigations directed at solving the problems described in the preceding paragraphs of this specification, the present inventors discovered that the timewise variation in curability can be suppressed by treating the finely divided silver particles with an organosilicon compound prior to combining the particles with the other ingredients of the curable organosiloxane composition.
The present inventors also discovered that the presence in the curable organosiloxane composition of an organosilicon compound containing silicon-bonded alkoxy groups that is in addition to the compound used to treat the silver particles, further reduces the timewise variations in contact resistance and volume resistivity of the cured elastomer. The present invention is the result of these two discoveries.
One objective of the present invention is to provide silver-filled silicone rubber composition that exhibits excellent values of electrical conductivity in combination with little timewise variation in curability and electrical conductivity. A second objective is to provide a method for treating silver particles intended for use in electrically conductive organosiloxane compositions.
The objectives of the present invention can be achieved by pretreating the silver particles intended for use in the present organosiloxane compositions with an organosilicon compound prior to combining the particles with the other ingredients of these compositions.
The variation in electrical properties with time exhibited by the curable composition can be further reduced by the presence in the curable composition of an alkoxy-containing organosilicon compound as an additive. This compound is in addition to any used as the organosilicon compound for treatment of the silver particles. An organohydrogensiloxane containing alkoxy groups will function both as this additive and the curing agent for the organosiloxane composition, or an organohydrogensiloxane and an organosilicon compound containing silicon-bonded alkoxy groups can be added as separate ingredients.
The present invention is described in Japanese patent application serial nos. 05/274,892 and 05/274,893, both filed on Oct. 6, 1993; and serial no. 05/311,265, filed on Nov. 17, 1993, on which Applicants base their claim to priority for the present application. The disclosures of these Japanese patent applications is hereby incorporated by reference into this specification.
The present invention provides an improved electrically conductive silicone rubber composition comprising
(A) 100 parts by weight of a polyorganosiloxane containing at least two alkenyl radicals per molecule,
(B) an organohydrogensiloxane containing at least two silicon-bonded hydrogen atoms in each molecule, in a quantity sufficient to provide from 0.5 to 3 silicon-bonded hydrogen atoms per alkenyl radical present in said polyorganosiloxane,
(C) from 50 to 2,000 parts by weight of finely divided silver particles, and
(D) a platinum-containing hydrosilation catalyst in a quantity sufficient to promote curing of said composition.
The improvement comprises treating the silver particles with an organosilicon compound selected from the group consisting of alkoxysilanes and organosiloxanes prior to combining the particles with the other ingredients of the present curable composition.
The variation in electrical properties with time of cured materials prepared using the present curable compositions can be reduced if the compositions contain up to 20 parts by weight of an organosilicon compound containing silicon-bonded alkoxy groups.
The Treated Silver Particles (Ingredient C)
The characterizing feature of the present curable organosiloxane compositions is the presence of silver particles that have been treated with an organosilicon compound prior to being combined with the other ingredients of the curable organosiloxane composition. In preferred embodiments, the organosilicon compound which is used to pre-treat the silver particles is selected from the group consisting of (i) silanes containing at least one alkoxy group and (ii) organosiloxanes.
The organosilicon compound used to treat the surface of the silver particles, referred to in this specification as ingredient C, is responsible for the electrical conductivity exhibited by silicone rubbers prepared by curing the present compositions, irrespective of the storage time of the curable composition used to prepare the elastomer.
The silver particles can be prepared by the chemical or electrolytic reduction of a silver compound such as silver nitrate, or by atomization of molten silver. The particles treated in accordance with the present invention can be 100 percent pure silver or a silver alloy. Useful silver alloys include silver/copper alloys and silver/palladium alloys. The silver alloys may also contain trace amounts of other metals such as zinc, tin, magnesium, and nickel.
With respect to non-metallic impurities present on the surface of the treated particles, to avoid inhibiting curing of the organosiloxane composition it is particularly preferred that the NH4+ content not exceed 10 ppm and that the SO42xe2x88x92 content not exceed 5 ppm.
While no specific restrictions apply to the diameter of the silver particles, average particle diameters in the range of from 0.1 to 10 micrometers are preferred. Because the morphology of ingredient C is likewise not critical, the particles can be in the form of granules, dendrites, flakes or the particles may be amorphous. Mixtures of silver particles exhibiting various morphologies can be used. The flake form of silver is preferred for the preparation of highly electrically conductive cured silicone elastomers.
The organosilicon compound used to treat the surface of the silver particles is not specifically restricted. Examples of suitable treating agents include but are not limited to:
alkoxysilanes such as methyltrimethoxysilane, vinyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, dimethyldimethoxysilane, trimethylmethoxysilane, trimethylethoxysilane, tetramethoxysilane, and tetraethoxysilane;
siloxane oligomers such as silanol-endblocked dimethylsiloxane oligomers, silanol-endblocked dimethylsiloxane/methylvinylsiloxane co-oligomers, silanol-endblocked methylvinylsiloxane oligomers, silanol-endblocked methylphenylsiloxane oligomers, 1,3,5,7-tetramethylcyclotetrasiloxane, and 1,3,5,7,9-pentamethylcyclopentasiloxane;
polyorganosiloxanes ranging from low-viscosity liquids to gums, and including but not limited to trimethylsiloxy-endblocked polydimethylsiloxanes, trimethylsiloxy-endblocked dimethylsiloxane/methylvinylsiloxane copolymers, trimethylsiloxy-endblocked dimethylsiloxane/methylphenylsiloxane copolymers, trimethylsiloxy-endblocked polymethylhydrogensiloxanes, trimethylsiloxy-endblocked dimethylsiloxane/methylhydrogensiloxane copolymers, silanol-endblocked polydimethylsiloxanes, silanol-endblocked dimethylsiloxane/methylvinylsiloxane copolymers, silanol-endblocked dimethylsiloxane/methylphenylsiloxane copolymers, silanol-endblocked polymethylhydrogensiloxanes, silanol-endblocked dimethylsiloxane/methylhydrogensiloxane copolymers, dimethylvinylsiloxy-endblocked polydimethylsiloxanes, dimethylvinylsiloxy-endblocked dimethylsiloxane/ methylvinyl-siloxane copolymers, dimethylvinylsiloxy-endblocked dimethylsiloxane/methylphenylsiloxane copolymers, dimethylhydrogensiloxy-endblocked polymethylhydrogensiloxanes, and dimethylhydrogensiloxy-endblocked dimethylsiloxane/methylhydrogensiloxane copolymers; and silicone resins, including but not limited to resins composed of R3SiO1/2 and SiO4/2 units, silicone resins composed of the RSiO3/2 unit, resins composed of the R2SiO2/2 and RSiO3/2 units, and resins composed of the R2SiO2/2, RSiO3/2, and SiO4/2 units .
The organosilicon compounds used to treat the silver particles can be used singly or as mixtures of two or more compounds. The group represented by R in the units of the silicone resins represents any of the substituted and unsubstituted monovalent hydrocarbon discussed in the section of this specification relating to the alkenyl-containing polyorganosiloxane, referred to in this specification as ingredient A.
Silicone resins used as silver treating agents in accordance with the present invention are preferably solids that soften at above room temperature, most preferably within the range from 50xc2x0 C. to 150xc2x0 C.
The thickness of the coating of the organosilicon compound or a polycondensation product of this compound that is formed during treatment of the silver particles is not critical. The conductivity of cured silicone elastomers prepared from the filled silicone rubber compositions is typically inversely proportional to the thickness of the coating, however thinner films reduce the affinity between the treated silver particles and the other ingredients of curable organosiloxane compositions, resulting in larger decreases with the passage of time in the curability of these compositions.
For these reasons, while the optimal coating thickness will be dependent upon the particular end-used application, thicknesses not exceeding 0.1 micrometer are preferred. In order to obtain highly electrically conductive cured silicone rubber, the excess organosilicon compound or polycondensation products of this compound should be removed from the surface of the silver particles by washing the particles with a suitable organic solvent.
The method for treating the silver particles with the organosilicon compound is not critical. As an example, the surface of the silver particles can be wetted with the compound in the absence of any solvent or with a solution of a liquid or solid organosilicon compound in a suitable organic liquid.
The present treatment method is suitable for silver particles prepared by chemical or electrolytic reduction or by atomization or other means for preparing finely divided particles of from molten silver and it alloys.
Chemically reduced silver particles can be prepared, for example, by the reduction of an aqueous silver nitrate solution with a chemical reducing agent such as hydrazine, formaldehyde, or ascorbic acid.
Electrolytically reduced silver is in the form of dendrites that are deposited on the cathode during the electrolysis of an aqueous silver nitrate solution.
Atomized silver particles can be prepared by spraying molten silver heated to at least 1,000xc2x0 C. into water or inert gas.
No specific restrictions apply to the technique for wetting the silver particles with the neat organosilicon compound or solutions of this compound in a suitable organic liquid. Suitable treatment techniques include but are not limited to spraying the silver particles with a neat or solubilized organosilicon compound, immersing the silver particles in a neat or solubilized organosilicon compound, and grinding the silver particles using the neat or solubilized organosilicon compound as a lubricant.
Methods involving grinding are preferred, because they produce a flake form of silver that is particularly suitable for preparing highly electrically conductive silicone rubber. The grinding process yields particularly desirable results for the silver flake product. During grinding of silver particles in the form of flakes the organosilicon compound functions not only as a surface-treatment agent, but it also functions to accelerate flake formation by becoming adsorbed onto the activated surface of the flakes, thereby inhibiting aggregation of the flakes into larger particles.
The device for grinding the silver particles is not critical. Useful devices for this purpose include but are not limited to stamping mills, ball mills, vibratory mills, hammer mills, roll mills, and the combination of a mortar and pestle.
The conditions for milling the silver particles are not specifically restricted. The conditions will be dependent at least in part on the diameter and shape of the silver particles. Grinding is preferably conducted while cooling the grinder due to the heat generated during this operation. The silver particles produced by this process are in the form of flakes that preferably have a diameter in the range of from 0.1 to 10 micrometers.
To facilitate formation of the desired thin coating of the organosilicon compound on the silver particles or when the compound has a relatively high viscosity, the compound(s) used to treat the particles is preferably dissolved in a suitable organic liquid. No specific restrictions apply to organic solvents usable for this purpose. Suitable solvents include but are not limited to alcohols such as methanol, ethanol, and isopropanol; aliphatic compounds such as hexane, heptane, and octane; alicyclic compounds such as cyclohexane and cyclooctane; aromatic compounds toluene and xylene; ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; and esters such as ethyl acetate, and Carbitol acetate.
No specific restrictions apply to the conditions for treating the surface of the silver particles with an organosilicon compound during the preparation of ingredient C. Treatment of the particles is preferably carried out at temperatures from ambient to 100xc2x0 C., preferably at least 50xc2x0 C., for periods from 24 to 150 hours.
To facilitate drying and also remove excess organosilicon compound adhering on the surface of the silver particles, the treated silver particles are preferably washed with any of the organic solvent listed in the preceding paragraph of this specification and then dried for at least 24 hours at temperatures from ambient to 105xc2x0 C.
The concentration of the treated silver particles (ingredient C) in the present compositions is typically from 50 to 2,000 parts by weight, preferably from 300 to 600 parts, per 100 parts by weight of the polyorganosiloxane referred to in this specification as ingredient A. When the concentration of ingredient C is less than 50 parts per 100 parts of ingredient A, there is a substantial decline in the electrical conductivity of the cured silicone rubber product.
When the concentration of ingredient C exceeds 2,000 weight parts per 100 weight parts of ingredient A there is typically a substantial decline in the fluidity of the corresponding composition and the ability to process the composition into a suitable cured elastomer becomes very problematic.
The Alkenyl-Substituted Polyorganosiloxane (Ingredient A)
The alkenyl-substituted polyorganosiloxane is referred to as ingredient A of the present curable compositions. This ingredient contains at least 2 alkenyl radicals in each molecule. Suitable alkenyl radicals include but are not limited to vinyl, allyl, butenyl, pentenyl, hexenyl, and heptenyl, with vinyl being preferred. The location of the alkenyl radicals in the molecules of ingredient A can be at the molecular chain terminals, on non-terminal silicon atoms, or at both positions.
No specific restrictions apply to the silicon-bonded organic groups other than alkenyl radicals. These groups are substituted or unsubstituted monovalent hydrocarbon radicals that include but are not limited to alkyl such as methyl, ethyl, propyl, butyl, pentyl and hexyl; aryl such as phenyl, tolyl and xylyl; aralkyl groups such as benzyl and phenethyl; and haloalkyl groups such as 3-chloropropyl, and 3,3,3-trifluoropropyl. Of these radicals, methyl and phenyl are preferred.
The molecular structure of ingredient A is also not critical and is specifically exemplified by straight chains, partially branched straight chains, branched chains, and network type resin structures. Straight-chain and partially branched straight-chain structures are preferred.
The viscosity of ingredient A is not critical. Typical polymers exhibit viscosity values, measured at 25xc2x0 C., in the range from 50 to 500,000 centipoise (0.05 to 500 Pa.s) while particularly preferred values fall in the range of from 400 to 100,000 centipoise (0.4 to 100 Pa.s)
Specific polyorganosiloxanes suitable for use as ingredient A include but are not limited to trimethylsiloxy-endblocked dimethylsiloxane/methylvinylsiloxane copolymers, trimethylsiloxy-endblocked polymethylvinylsiloxanes, trimethylsiloxy-endblocked methylvinylsiloxane/methylphenylsiloxane copolymers, trimethylsiloxy-endblocked dimethylsiloxane/methylvinylsiloxane/methylphenylsiloxane copolymers, dimethylvinylsiloxy-endblocked polydimethylsiloxanes, dimethylvinylsiloxy-endblocked polymethylvinylsiloxanes, dimethylvinylsiloxy-endblocked polymethylphenylsiloxanes, dimethylvinylsiloxy-endblocked dimethylsiloxane/methylvinylsiloxane copolymers, dimethylvinylsiloxy-endblocked dimethylsiloxane/methylphenylsiloxane copolymers, silanol-endblocked dimethylsiloxane/methylvinylsiloxane copolymers, silanol-endblocked polymethylvinylsiloxanes, and silanol-endblocked dimethylsiloxane/methylvinylsiloxane/methylphenylsiloxane copolymers.
Alkenyl-substituted organosiloxane resins suitable for use as ingredient A include but are not limited to resins composed of the combination R3SiO1/2 and SiO4/2 units, the RSiO3/2 unit alone, the combination of R2SiO and RSiO3/2 units, the combination of R2SiO, RSiO3/2 and SiO4/2 units, and mixtures containing two or more of these resins.
The monovalent hydrocarbon radicals represented by R in the preceding formula can be substituted or unsubstituted, and include but are not limited to alkyl such as methyl, ethyl, propyl, butyl, pentyl and octyl; alkenyl such as vinyl, allyl, butenyl, pentenyl and hexenyl; aryl such as phenyl, tolyl and xylyl, and haloalkyl such as 3-chloropropyl and 3,3,3-trifluoropropyl. The only proviso is that at least one of the R groups represents an alkenyl radical.
The Organohydrogensiloxane (Ingredients B and Bxe2x80x2)
The organohydrogensiloxanes referred to in this specification as ingredients B and Bxe2x80x2 function as crosslinkers that are responsible for curing of the present compositions. The organohydrogensiloxane should contain at least 2 silicon-bonded hydrogen atoms in each molecule. The location of these silicon-bonded hydrogen atoms is not critical, and they may be located, for example, at the molecular chain terminals or on non-terminal silicon atoms or at both positions.
No specific restrictions apply to the silicon-bonded organic groups in ingredient B, which are substituted and unsubstituted monovalent hydrocarbon radicals. Specific radicals are listed in the preceding section of this specification relating to ingredient A, with the exception that alkenyl and other ethylenically unsaturated radicals are excluded.
As discussed in connection with ingredient A of the present compositions, the molecular structure of ingredient B is likewise not critical and is specifically exemplified by straight chain, partially branched straight chain, branched, and network. Straight-chain and partially branched straight-chain structures are preferred.
The viscosity of ingredient B critical is not critical, however preferred viscosity values, measured at 25xc2x0 C. are in the range of from 1 to 50,000 centipoise, (0.001 to 50 Pa.s) with particularly preferred values being in the range of 5 to 1,000 centipoise (0.005 to 1 Pa.s).
Examples of organohydrogensiloxanes suitable for use as trimethylsiloxy-endblocked polymethylhydrogensiloxanes, trimethylsiloxy-endblocked dimethylsiloxane/methylhydrogensiloxane copolymers, trimethylsiloxy-endblocked methylhydrogensiloxane/methylphenylsiloxane copolymers, trimethylsiloxy-endblocked dimethylsiloxane/methylhydrogensiloxane/methylphenylsiloxane copolymers, dimethylhydrogensiloxy-endblocked polydimethylsiloxanes, dimethylhydrogensiloxy-endblocked polymethylhydrogensiloxanes, dimethylhydrogensiloxy-endblocked dimethylsiloxane/methylhydrogensiloxane copolymers, dimethylhydrogensiloxy-endblocked dimethylsiloxane/methylphenylsiloxane copolymers, dimethylhydrogensiloxy-endblocked polymethylphenylsiloxanes, silanol-endblocked polymethylhydrogensiloxanes, silanol-endblocked dimethylsiloxane/methylhydrogensiloxane copolymers, silanol-endblocked methylhydrogensiloxane/methylphenylsiloxane copolymers, and silanol-endblocked dimethylsiloxane/ methylhydrogensiloxane/methylphenylsiloxane copolymers.
In an alternative embodiment of ingredient B, referred to as ingredient Bxe2x80x2, the organohydrogensiloxane functions both as an adhesion promoter and as a crosslinker for the curable composition. Each molecule of ingredient Bxe2x80x2 must contain at least 2 silicon-bonded hydrogen atoms and at least 1 silicon-bonded alkoxy group. The location of the silicon-bonded hydrogen atoms in ingredient Bxe2x80x2 is not critical. These hydrogen atoms may be bonded, for example, to terminal or non-terminal silicon atoms or at both of these positions. The bonding position for the silicon-bonded alkoxy groups in ingredient Bxe2x80x2 is also not critical, and this group may be bonded to terminal and/or non-terminal silicon atoms.
The silicon-bonded organic groups that can be present in ingredient Bxe2x80x2 are free of ethylenic unsaturation and are specifically exemplified by but not limited to the monovalent substituted and unsubstituted hydrocarbon radicals that can be present in ingredient B. Ingredient Bxe2x80x2 can contain other adhesion-promoting groups that will not interfere with curing of the present compositions. These additional adhesion-promoting groups include but are not limited to epoxy groups that are bonded to a silicon atom by means of a carbon atom that is not part of the epoxide ring.
The molecular structure of ingredient Bxe2x80x2 includes but is not limited to straight chains, partially branched straight chains, branched chains, cyclic and network structures. Mixtures of polyorganosiloxanes having two or more types of structures can be used. While the viscosity of ingredient Bxe2x80x2 is not critical, viscosities of from 1 to 50,000 centipoise (0.001 to 50 Pa.s), measured at 25xc2x0 C., are preferred, the range from of 5 to 1,000 centipoise (0.005 to 1 Pa.s) being particularly preferred.
Polyorganosiloxanes suitable for use as ingredient Bxe2x80x2 include but are not limited to the following structures. 
In the foregoing formulae a represents an integer with a value of at least 1, and c represents an integer with a value of at least 2.
No particular restrictions apply to the method for preparing ingredient Bxe2x80x2. Suitable methods include but are not limited to
(a) the platinum-catalyzed addition of an alkenyl-containing alkoxysilane to a portion of the silicon-bonded hydrogen atoms of an organohydrogensiloxane containing at least 3 silicon-bonded hydrogen atoms in each molecule, and
(b) the platinum-catalyzed addition of an alkenyltrialkoxysilane and an alkenyl-containing epoxy compound to a portion of the silicon-bonded hydrogen atoms of an organohydrogensiloxane containing at least 4 silicon-bonded hydrogen atoms in each molecule.
Platinum catalysts useful for preparing ingredient Bxe2x80x2 include the same catalysts described in the preceding section of this specification relating ingredient D. Polyorganosiloxanes containing at least three or four silicon-bonded hydrogen atoms and suitable for use in preparing ingredient Bxe2x80x2 include but are not limited to trimethylsiloxy-endblocked polymethylhydrogensiloxanes, trimethylsiloxy-endblocked dimethylsiloxane- methylhydrogensiloxane copolymers, dimethylhydrogensiloxy-endblocked polymethylhydrogensiloxanes, dimethylhydrogensiloxy-endblocked dimethylsiloxane-methylhydrogensiloxane copolymers, cyclic methylhydrogensiloxanes, and cyclic dimethylsiloxane-methylhydrogensiloxane copolymers.
Alkenyl-containing alkoxysilanes suitable for use in preparing ingredient Bxe2x80x2 include but are not limited to vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane, vinyldimethylmethoxysilane, allyltrimethoxysilane, allyltriethoxysilane, allylmethyldimethoxysilane, allyldimethylmethoxysilane, and butenyltrimethoxysilane.
Alkenyl-containing epoxy compounds suitable for use in preparing ingredient Bxe2x80x2 include but are not limited to vinyl glycidyl ether, allyl glycidyl ether, butenyl glycidyl ether, 3,4-epoxycyclohexylethene, 3-(3,4-epoxycyclohexyl)propene, and 4-(3,4-epoxycyclohexyl)butene.
Because ingredient Bxe2x80x2 must contain at least two silicon bonded hydrogen atoms per molecule, the total number of moles of alkenyl-containing alkoxysilane and alkenyl-containing epoxy compound used to prepare this ingredient must be at least two less than the number of moles of silicon-bonded hydrogen present in the initial organohydrogensiloxane.
The concentration of ingredients B and Bxe2x80x2 in the present compositions is sufficient to provide from 0.5 to 3 silicon-bonded hydrogen atoms per alkenyl group in ingredient A. The composition will not cure adequately when ingredients B and Bxe2x80x2 provide fewer than 0.5 silicon-bonded hydrogen atoms per alkenyl radical in ingredient A. At the other extreme, the presence in the curable composition of more than 3 silicon-bonded hydrogen atoms per alkenyl group in ingredient A yields a cured electrically conductive silicone rubber with a drastically reduced heat resistance.
The Platinum-Containing Hydrosilation Catalyst (Ingredient D)
The platinum containing hydrosilation catalyst, also referred to in this specification as ingredient D, accelerates the cure of the present compositions. Any metal from the platinum group of the periodic table capable of functioning as catalysts for hydrosilation reactions can be used as ingredient D. Suitable catalysts include but are not limited to platinum black, platinum supported on powdered alumina, platinum supported on powdered silica, platinum supported on powdered carbon, chloroplatinic acid, alcohol solutions of chloroplatinic acid, chloroplatinic acid/olefin complexes, chloroplatinic acid/vinylsiloxane complexes, and platinum catalysts dispersed in microparticulate forms of thermoplastic organic resins such as methyl methacrylate resins, polycarbonate resins, polystyrene resins, and silicone resins.
The concentration of ingredient D in the present compositions is typically not critical so long as it is sufficient to promote curing of the composition, and is typically equivalent to from 1 to 100 ppm of platinum metal, based on the combined weights of ingredients A and B.
The Optional Alkoxy-Functional Organosilicon Compound (Ingredients E and Exe2x80x2)
Curable compositions of the present invention can contain only ingredients A-D, however an organosilicon compound containing at least one silicon-bonded alkoxy group per molecule, referred to in this specification as ingredients E and Exe2x80x2, is preferably included in the present compositions to diminish the timewise variations in contact resistance and volume resistivity values exhibited by cured materials prepared from these compositions. Ingredient E is used when ingredient B is present as the organohydrogensiloxane and ingredient Exe2x80x2 is used in combination with ingredient Bxe2x80x2.
Examples of suitable organosilicon compounds suitable for use as ingredient E include but are not limited to alkoxysilanes such as tetramethoxysilane, tetraethoxysilane, dimethyldimethoxysilane, methylphenyldimethoxysilane, methylphenyldiethoxysilane, phenyltrimethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, vinyltrimethoxysilane, allyltrimethoxysilane, allyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane and organosilicon compounds with the following formulae: 
wherein a and b are each integers with values of at least 1 
where a is an integer with a value of at least 1 and c is 0 or 1.
Ingredient Exe2x80x2 can be present as an optional ingredient when ingredient Bxe2x80x2 is used as the organohydrogensiloxane. Ingredient Exe2x80x2 is an organosilicon compound that contains silicon-bonded alkoxy groups and either 1 or no silicon-bonded hydrogen. Ingredient Exe2x80x2 can be any of the organosilicon compounds suitable for use as ingredient E that contain a maximum of one silicon-bonded hydrogen atom.
When they are present, the concentrations of ingredients E and Exe2x80x2 are up to 20 weight percent, based on the weight of ingredient A, preferably from 0.5 to 8 weight percent. The appearance of timewise variations in the contact resistance and volume resistivity values of the cured elastomer becomes a possibility when ingredient E or Exe2x80x2 is not present. On the other hand, the addition of more than 20 weight percent of ingredient E or Exe2x80x2 based on ingredient A results in a decline in the storage stability of the resulting composition and also in an increase in the hardness of the cured elastomer with elapsed time.
Other Optional Ingredients
A cure inhibitor may also be added to the instant composition as an optional ingredient to improve the storage stability and handling characteristics of the curable composition. Suitable cure inhibitors include but are not limited to alkynyl alcohols such as 3-methyl-1-butyn-3-ol, 3,5-dimethyl-1-hexyn-3-ol and phenylbutynol; ene-yne compounds such as 3-methyl-3-penten-1-yne and 3,5-dimethyl-3-hexen-1-yne; and 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane, 1,3,5,7-tetramethyl-1,3,5,7-tetrahexenylcyclotetrasiloxane, and benzotriazole.
These cure inhibitors are preferably added at 0.001 to 5 weight parts per 100 weight parts of ingredient A.
The present curable compositions can also include an inorganic filler to impart a suitable hardness and strength to the cured elastomer. Suitable inorganic fillers include but are not limited to fumed silica, crystalline silica, calcined silica, wet-process silica, fumed titanium oxide, and carbon black, and by inorganic filler whose surface has been treated with an organosilicon compound such as an organoalkoxysilane, organochlorosilane or an organodisilazane.
These inorganic fillers are preferably added at no more than 50 weight parts per 100 weight parts of ingredient A.
The elastomers prepared using the present curable organosiloxane compositions typically exhibit volume resistivities below 0.1 ohm-cm, preferably below 1xc3x9710xe2x88x923 ohm-cm, and are useful as electrically conductive adhesives, electrically conductive die-bonding agents, as heat-dissipating die-bonding agents, and as electromagnetic-shielding agents.
The following examples describe preferred compositions of the present invention and electrically conducting elastomers prepared using these compositions, and should not be interpreted as limiting the scope of the present invention as defined in the accompanying claims. Unless otherwise specified all parts and percentages are by weight and reported viscosity values were measured at 25xc2x0 C.
The following methods were used to measure the various properties of the curable organosiloxane compositions and the electrically conductive cured elastomers prepared using these compositions.
Appearance of the Electrically Conductive Silicone Rubber Compositions
The electrically conductive silicone rubber compositions were stored in transparent glass bottles that were maintained under refrigeration. The appearance of the composition was inspected immediately after preparation (initial evaluation) and following 1 month, 3 months, and 6 months storage.
Curability of the Compositions
The curable electrically conductive composition were stored under refrigeration after being prepared. Samples were taken from the compositions immediately following their preparation (initial evaluation) and after 1, 3, and 6 months, and cured elastomers were prepared from these samples by heating them for 30 minutes at 150xc2x0 C. The curability of the compositions was evaluated by measuring the hardness of the cured elastomers using a JIS A hardness meter in accordance with JIS K 6301.
Contact Resistance of the Silicone Rubbers
Each of the curable compositions was coated on one surface of a circuit board and heated for 30 minutes at 150xc2x0 C. to produce a cured elastomer. The contact resistance of the silicone rubber was then measured using the xe2x80x9c4-point methodxe2x80x9d to give the initial value. The contact resistance of the silicone rubber was measured by the same method after the elastomer-coated circuit board had been heated for 100 hours, 500 hours, and 1,000 hours in an oven maintained at 150xc2x0 C.
Volume Resistivity of the Silicone Rubbers
An electrically conductive elastomer in the form of a sheet with a thickness at least 1 mm was prepared by heating the curable silver-filled organosiloxane compositions at 150xc2x0 C. for 30 minutes. The initial volume resistivity of this silicone rubber sheet was measured using a model K-705RL meter from Yugen Kaisha Kyowa Riken). In order to measure the timewise variation in the volume resistivity of the silicone rubber, the volume resistivity of each of the elastomer sheets was measured by the same method after the sheets had been held for 100 hours, 500 hours, and 1,000 hours in a 150xc2x0 C. oven.
Adhesion of the Cured Elastomers
The adhesion of the electrically conductive silicone elastomers was measured using a tab bonding test. In accordance with this test method, the curable compositions to be evaluated were heated for 30 minutes at 150xc2x0 C. on an aluminum plate to form a silicone rubber bead measuring 20 mm in width, 20 mm in length and 5 mm in thickness. This cured elastomeric bead was then peeled from the aluminum plate, and the surface that had been in contact with the heated plate was examined. A rating of xe2x80x9cCFxe2x80x9d indicate the occurrence of cohesive failure within the body of the elastomer, leaving the plate covered with cured elastomer. A rating of xe2x80x9cAFxe2x80x9d refers to adhesive failure that occurred only at the interface between the elastomer and the aluminum heating surface. A rating of xe2x80x9cpartial AFxe2x80x9d indicates only partial interfacial failure and some within the body of the cured elastomer, and a score of xe2x80x9cTCFxe2x80x9d refers to the presence of a thin layer of silicone rubber adhering to the aluminum plate.
The evaluation results for curable compositions and elastomers of the present invention are recorded in Table 1 and the evaluation results for the comparative examples are recorded in Table 2.